During many chronic viral infections, virus-specific CD8 T cells become exhausted as a result of prolonged antigenic and inflammatory stimulation. Exhausted CD8 T cells are characterized by a progressive loss of the ability to produce effector cytokines and kill target cells, poor proliferative capacity, and increased expression of multiple inhibitory receptors. T cell exhaustion was first characterized in the murine model of lymphocytic choriomeningitis virus (LCMV) infection. Considerable effort has been focused on elucidating the mechanisms that regulate T cell exhaustion, with multiple factors and pathways implicated. Blockade of the PD-1:PD-L1 inhibitory pathway early during infection can prevent progression of CD8 T cell exhaustion and, if given at later time points, can partially rescue the function of exhausted CD8 T cells. Recent results show that PD-L1 blockade does not reinvigorate all exhausted CD8 T cells but selectively expands a subset of exhausted CD8 T cells into effectors that function only temporarily, reverting back to the exhausted state. Even when removed from the chronic infection environment, exhausted CD8 T cells are unable to differentiate into functional memory, indicating that the functional defects are molecularly imprinted within exhausted T cells. Our recent work has shown that progressive loss of CD8 T cell functionality during chronic LCMV infection is associated with decreased diacetylated histone H3 (diAcH3) levels globally and at specific loci encoding effector cytokines, indicating a loss of open chromatin structure at these loci. In vitro treatment of exhausted T cells with valproic acid (VPA), a histone deacetylase inhibitor (HDACi), restores diAcH3 levels and effector cytokine production. Remarkably, when adoptively transferred into nave hosts, VPA-treated ?exhausted? CD8 T cells exhibit improved functionality for an extended period of time and differentiate into functional memory T cells with enhanced recall responses. Together, these results show that epigenetics plays a crucial role in regulating T cell exhaustion and may hold the key to reprogramming exhausted T cells into functional effectors capable of clear chronic infection. In this application, we will test 1) if adoptive transfer of exhausted CD8 T cells that have been rescued by in vitro HDACi treatment will work in vivo to clear an established chronic LCMV infection, and 2) if direct treatment of chronically infected mice with HDACi in vivo can rejuvenate exhausted CD8 T cell and allow for viral clearance. We will examine how exhausted T cells are reprogrammed epigenetically and transcriptionally by HDAC inhibition to understand the mechanisms of HDACi in rescuing exhaustion. We will test whether HDACi can reprogram and rescue subsets of exhausted CD8 T cells that fail to respond to PD-L1 blockade. We will examine if combining PD-L1 blockade with epigenetic manipulation synergizes to restore the functionality of all exhausted CD8 T cells and achieve faster viral clearance. Through these studies, we hope to gain a better understanding of the molecular mechanisms underlying T cell exhaustion and to develop an optimal strategy for enhancing immune control and clearance of chronic viral infection.